A contrivance of this general type is, for example, known from Journal "Tecnica Molitoria", No. 6 of Mar. 30, 1963, page 133/136 and from Swiss Patent CH-PS 587 687. In the case of these contrivances, the two fluidized bed tables are arranged parallel and rigidly joined together by a common housing. A common oscillating drive induces in the fluidized bed tables an oscillatory casting motion that is the same in direction, amplitude and frequency. If the inclination of the fluidized bed tables and the oscillatory casting motion are optimally tuned to the lower fluidized bed table, disturbance of material flow in the upper fluidized bed table can occur. If the inclination of the fluidized bed tables and the oscillatory casting motion are optimally tuned to the upper fluidized bed table, there then exists the danger that larger quantities of stones will be carried along with the remaining grain product.
The Japanese Patent Sho-35-13622 shows another similar contrivance, with two air-penetrable separation tables capable of being oscillated in relationship to a frame, forming an oscillatory unit with a common vibrator. As is given in this publication, three different fractions are formed over appropriate outlets whereby, however, both a fine and coarse fraction of impurities (dust and sand) with only one fraction of grain product is formed. This solution would be capable of being used in the preparation of cereals only for obtaining two grain fractions when there would be two grain parts of different size in the cereal that are capable of being separated by a purely sifting process. This seldom occurs in the case of cereals. Most often, a light grain fraction (shriveled kernels, etc.) must be separated from the good kernels. The shriveled kernels are not very different from other kernels in outside dimensions: they are, however, much lighter.
The actual standard for sorting out heavier impurities from the cereals is given, today, by solutions corresponding to Swiss Patent CH-PS 491 685. In the separating contrivance in accordance with this publication, also used, in principle, are two separation tables: here, however, these are next to each other. The product is layered into a heavy and light fraction in a first air-penetrable channel, and then the two layers are passed over to the second separating table for separation into a heavy fraction and a light fraction. In the greater number of cases, however, sorted out as the heavy fraction are only the stones, glass, etc., so that here also, as in the case of the Japanese Patent Sho-35-13622, only one cereal fraction is actually obtained.
As a general rule, no absolute values can be achieved in practice; this applies particularly for the separation of grain product and/or cereals into several fractions. The practical maximum in the case of sorting out stones is assumed to be in a flowthrough water bath, since, in water, the difference of weight between cereal grain and stones can be utilized especially well because the stones simply sink to the bottom. Actually, however, as good a result as with the water bath is achieved with the solution in accordance with Swiss Patent CH-PS 491 685.
For separating the cereal into two fractions, a heavy grain fraction and a light grain fraction, two contrivances have been used: the so-called Concentrator in accordance with Swiss Patent CH-PS 547 667 for example, and the light grain sorter in accordance with Swiss Patent CH-PS 527 002.
Although an attempt has been made for decades to sort out the three principal fractions from cereals; namely heavy grain, light grain and stones, with a single machine, up until the present time no solutions have been found that achieve the quality standard of the contrivances in accordance with Swiss Patents CH-PS 491 685, 547 667 and 527 002.